1. Field of the Invention
Aspects of the present disclosure relate to electrolytes for lithium secondary batteries and lithium secondary batteries including the electrolytes, and more particularly, to electrolytes for lithium secondary batteries that improve high temperature lifetime characteristics and high temperature conservation characteristics of the battery as well as lithium secondary batteries including the electrolytes.
2. Description of the Related Art
Lithium secondary batteries are rechargeable at a high speed, and have an energy density per unit weight that is at least three times greater than that of existing lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries, or nickel-zinc batteries. In addition, the charging rates of lithium secondary batteries are high. Due to such features, research and development of lithium secondary batteries are actively being performed.
In general, a lithium secondary battery includes a positive electrode, a negative electrode, a separator and an electrolyte disposed between the positive electrode and the negative electrode. In particular, a method of improving performance of a lithium secondary battery by adding a small amount of additives to an electrolyte without largely affecting physical properties of the electrolyte draws much attention.
Electrolyte additives have various functions. For example, the additive may form a solid electrolyte interface (SEI) for preventing direct contact between an electrode active material and the electrolyte. Additives for forming an SEI on the surface of an electrode can be classified as an anode additive for helping to form an SEI on the surface of graphite and an overcharge protection (OCP) additive for forming a thick film on the surface of a positive electrode.
As recent demand for lithium secondary batteries having high energy density, for example, batteries for electric vehicles, increases, research into high voltage positive active materials has been conducted. However, research into an electrolyte additive for preventing oxidation of an electrolyte occurring at the surface of a cathode active material has not yet been implemented.
In general, the potential window of an electrolyte needs to be wider than the potential difference between the positive active material and the negative active material. However, as an active material for high voltage is used to increase the energy density of a battery, the potential window of the electrolyte becomes narrower than that between the positive active material and the negative active material. Accordingly, decomposition of the electrolyte may be prevented by forming a film for preventing direct contact between the electrolyte and the electrode active material.
If aromatic compounds such as biphenyl and terphenyl are used as electrolyte additives, the electrolyte additive functions as an OCP by forming a thick film at the surface of the cathode when the voltage of the battery is equal to or higher than a reference voltage value so as to block passage of lithium ions and current flow. Recently, a method of forming a thin film at the surface of a cathode by using a low concentration of an additive has been introduced. However, the battery characteristics obtained are not satisfactory and thus there is still plenty of room for improvement.